This invention relates to shaped refractory metal boride articles. An important metal boride is titanium diboride TiB.sub.2, which has been proposed for use in electrolytic aluminium reduction cells on account of its electrical conductivity and resistance to corrosion, and of the fact that it is wetted by molten aluminium metal but not by molten cryolite. But the invention is applicable also to other refractory metal borides.
Shaped articles of titanium diboride are conventionally produced by hot pressing titanium diboride powder or cold pressing followed by sintering. These operations are labour and energy intensive, and the sintering requires temperatures in excess of 2000.degree. C.
One way of making titanium diboride powder is by briquetting boron carbide B.sub.4 C, carbon, and rutile titanium dioxide TiO.sub.2 into pellets and heating to form titanium diboride. The reacted pellets are then ground and the resulting powder shaped and sintered. Since titanium diboride is a very hard metal, grinding is an expensive operation and introduces impurities from the grinding media and the atmosphere. Further impurities are introduced during powder handling, pressing and sintering operations. During sintering, the titanium diboride grain size grows, typically to the 50-100 micron range. By reason of this coarse grain structure and of the impurities, the shaped sintered products are subject to grain boundary attack, cracking, corrosion and disintegration when exposed to the sort of conditions that are found in electrolytic aluminium reduction cells.
U.S. Pat. No. 4,108,670 describes a titanium diboride article in the form of a dense matrix having a grain size of 1-20 microns and having from 5-50% by volume of large pores from 4-1000 microns diameter, many of which are isolated. The article is made by sintering from titanium boride formed as a sub-micron powder, so the need for grinding is avoided. But the powder handling, pressing and sintering operations necessarily introduce impurities. In particular, the article contains 0.1-5% by weight of carbon, added as a sintering aid. However, the presence of free carbon is disadvantgeous because it reacts to form Al.sub.4 C.sub.3 and the associated volume expansion causes cracking.